1. Field of the Invention
This invention relates to a focus detecting device suitable for a photographic camera, a video camera or the like, and in particular to a focus detecting device suitable for dividing the pupil of an objective lens into a plurality of areas, forming light intensity distributions regarding a plurality of object images by the use of light beams passing through said areas, and finding the relative positional relation between the plurality of light intensity distributions to thereby detect the in-focus state of the objective lens.
2. Related Background Art
As a passive type focus detecting system utilizing a light beam passing through an objective lens, there is the so-called correlation system.
This correlation system is well known, for example, from Japanese Laid-Open Patent Application No. 59-107311, Japanese Laid-Open Patent Application No. 59-107313, etc.
FIG. 13 of the accompanying drawings is a schematic view of the optical system of a focus detecting device using the prior-art correlation system.
In FIG. 13, the reference numeral 61 designates an objective lens, and the reference numeral 62 denotes a field mask disposed near the predetermined imaging plane of the objective lens 61. The reference numeral 63 designates a field lens disposed near the predetermined imaging plane. The reference numeral 64 denotes a secondary optical system comprised of two lenses 64-1 and 64-2 disposed symmetrically with respect to the optic axis of the objective lens 61. The reference numeral 65 designates light receiving means having two light receiving element arrays 65-1 and 65-2 disposed rearwardly of the two lenses 64-1 and 64-2 correspondingly thereto. The reference numeral 66 denotes a stop having two opening portions 66-1 and 66-2 disposed rearwardly of the two lenses 64-1 and 64-2 correspondingly thereto. The reference numeral 67 designates the exit pupil of the objective lens 61 which is comprised of two divided areas 67-1 and 67-2.
The field lens 63 has the function of imaging the opening portions 66-1 and 66-2 on the areas 67-1 and 67-2, respectively, of the exit pupil 67, and light beams passed through the areas 67-1 and 67-2 may form light intensity distributions on the light receiving element arrays 65-1 and 65-2, respectively. It is to be understood that the light includes not only visible light but also invisible light.
In the focus detecting device shown in FIG. 13, where the imaging point of the objective lens 61 is forward of the predetermined imaging plane, the light intensity distributions regarding the object images formed on the two light receiving element arrays 65-1 and 65-2, respectively, become close to each other, and where the imaging point of the objective lens 61 is rearward of the predetermined imaging plane, the light intensity distributions formed on the two light receiving element arrays 65-1 and 65-2, respectively, become far from each other. Moreover, the amount of deviation between the light intensity distributions formed on the two light receiving element arrays 65-1 and 65-2, respectively, is in a certain functional relation with the amount of out-of-focus of the objective lens 61 and therefore, if that amount of deviation is calculated by suitable calculating means, the direction and amount of out-of-focus of the objective lens 61 can be detected.
The focus detecting device shown in FIG. 13 is effecting distance measurement for an object existing substantially centrally of the object range photographed by the objective lens.
In contrast, a focus detecting device capable of accomplishing focus detection with respect to any other measuring point than the central portion of the photographing range has previously been proposed by the applicant in Japanese Patent Application No. 62-279835.
FIG. 14 of the accompanying drawings is a schematic view of the optical system of a focus detecting device for a plurality of distance measuring points proposed in Japanese Patent Application No. 62-279835. In FIG. 14, the reference numeral 71 designates a field mask, the reference numeral 72 denotes a field lens, the reference numeral 73 designates a stop having two openings 73-1 and 73-2, the reference numeral 74 denotes a secondary optical system comprising two lenses 74-1 and 74-2, and the reference numeral 75 designates a sensor. The objective lens 61 shown in FIG. 13 is not shown in FIG. 14.
In FIG. 14, the field mask 71 has a plurality of openings 71a-71e correspondingly to a plurality of fields of view to be focus-detected, and pairs of sensor arrays 75a1 and 75a2, 75b1 and 75b2, 75c1 and 75c2, 75d1 and 75d2, and 75e1 and 75e2 are provided as the sensor unit 75 so as to receive pairs of light intensity distributions which the light beams controlled by the field mask 71 form by the secondary optical system 74.
In FIG. 14, detection is effected in five areas, i.e., the central portion of the photographing picture plane and four locations on both sides thereof. It is very important for applying the focus detecting device to a camera that focus detection can be effected in a plurality of areas in the photographing picture plane by such a simple construction.
In the focus detecting device shown in FIG. 14, depending on the focus state of the objective lens, the direction in which the two light intensity distributions on the sensor move relative to each other is a vertical direction and therefore, detection is possible only for an object having a variation in the light intensity distribution in this direction, and accuracy of distance measurement may be reduced for an object having a variation in the light intensity distribution only in a direction perpendicular to said direction, such as, for example, a black and white edge pattern with a vertical line as a boundary.
Therefore, the applicant has proposed in Japanese Patent Application No. 63-274940 a focus detecting device which can accomplish distance measurement even for an object whose light intensity distribution varies only in one direction, i.e., a vertical or horizontal direction, near the center of the photographing range and moreover, can accomplish detection even at a plurality of other points than the vicinity of the center of the photographing range.
FIG. 15 of the accompanying drawings shows the essential portions of the focus detecting device proposed in Japanese Patent Application No. 63-274940.
In FIG. 15, the reference numeral 31 designates a field mask having, for example, a cruciform opening 31-1 intersecting substantially at the center of the photographing picture plane of an objective lens (a photo-taking lens), not shown, and vertically long openings 31-2 and 31-3 on both sides of the cruciform opening 31-1. The reference numeral 32 denotes a field lens comprising three areas 32-1, 32-2 and 32-3 each having a predetermined optical characteristic correspondingly to the three openings 31-1, 31-2 and 31-3 in the field mask 31. The reference numeral 33 designates a stop having a vertical pair of openings 33-1a and 33-1b and a horizontal pair of openings 33-1c and 33-1d in the central portion thereof, and two pairs of openings 33-2a and 33-2b, 33-3a and 33-3b in the marginal portion thereof. The areas 32-1, 32-2 and 32-3 of the field lens 32 have the function of imaging the openings 33-1, 33-2 and 33-3 in the stop 33 which form respective pairs near the exit pupil of the photo-taking lens not shown. The reference numeral 34 denotes a secondary optical system which, as a whole, has four pairs of secondary imaging lenses. That is, the secondary optical system 34 as a whole comprises eight secondary imaging lenses 34-1a, 34-1b, 34-1c, 34-1d, 34-2a, 34-2b, 34-3a and 34-3b disposed rearwardly of the openings in the stop 33 correspondingly thereto.
The reference numeral 35 designates a light receiving element unit (a sensor unit) which, as a whole, has four pairs of sensor arrays. That is, the light receiving element unit 35 as a whole comprises eight sensor arrays 35-1a, 35-1b, 35-1c, 35-1d, 35-2a, 35-2b, 35-3a and 35-3b disposed correspondingly to the secondary imaging lenses so as to receive the images formed thereby.
FIG. 16 of the accompanying drawings illustrates image areas formed on the surface of the sensor 35 of FIG. 15. Areas 36-1a, 36-1b, 36-1c and 36-1d are the image areas of the central opening 31-1 in the field mask 31, and show a state in which a light beam transmitted through the central portion 32-1 of the field lens 32 is controlled by the openings 33-1a, 33-1b, 33-1c and 33-1d, and thereafter is formed on the surface of the sensor 35 by the secondary imaging lenses 34-1a, 34-1b, 34-1c and 34-1d rearward of said openings. The reference characters 36-2a and 36-2b denote the image areas of the opening 31-2 in the marginal portion of the field mask 31, and these image areas show a state in which a light beam transmitted through the marginal portion 32-2 of the field lens 32 is controlled by the openings 33-2a and 33-2b in the stop 33, and thereafter is fomred on the sensor unit 35 by the secondary imaging lenses 34-2a and 34-2b rearward of said openings. Likewise, openings 36-3a and 36-3b are the image areas of the opening 31-3 in the marginal portion of the field mask 31, and show image areas in which a light beam transmitted through the marginal portion 32-3 of the field lens 32 is controlled by the openings 33-3a and 33-3b in the stop 33, and thereafter is formed on the surface of the sensor 35 by the secondary imaging lenses 34-3a and 34-3b rearward of said openings.
The focus detecting device shown in FIG. 15 is designed such that focus detection can be accomplished in the center of the field of view in the photographing range as well as in the other areas than the center of the field of view, and this has led to the tendency that the secondary optical system, the light receiving means, etc. become relatively bulky due to the relation with the length of the optical path leading from the secondary optical system to the light receiving means.
Also, it is desirable from the point of detection accuracy to set the imaging magnification of the secondary optical system to a suitable value (e.g. about 1/3 time), but according to this, the position at which the secondary optical system is disposed is determined almost primarily by the full length of the focus detecting system and therefore, the degree of freedom of the arrangement of the various optical elements has been limited.